The invention relates to a magnetic resonance system that performs, by utilizing magnetic resonance phenomena, such measurements as spin density distributions, relaxation time constant distributions and spectroscopy of particular kinds of nuclei contained within a body to be examined. Such nuclei as hydrogen and phosphorus may be examined from outside the body in a non-invasive fashion so as to obtain cross-sectional image information of the desired measurements.
A conventional magnetic resonance system is shown in FIG. 11. A body to be examined, i.e., a patient 1, is fixedly placed on a bed 2. In the periphery surrounding the patient there is disposed a RF coil (Radio Frequency transmission/reception coil) 3, and in the outer periphery thereof, are additionally disposed a shim coil 4 and a gradient coil 5. All these coils are incorporated within a room temperature bore 7 (usually with an inner diameter of approximately 1 meter) of a large-size magnet 6 for whole body scanning. The whole body magnet 6 may be fabricated from a superconducting magnet, a normally-conducting magnet or a permanent magnet.
The magnet 6 is energized and de-energized through a current lead 9 by an excitation power source 8. (In the case of a permanent magnet, such energization is not necessary.) In the case of a superconductor magnet, the current 9 is usually removed after energization and subsequent production of the desired magnetic field. For superconducting magnets, a perpetual current is established, and the current 9 is generally removed to reduce consumption of liquid helium disposed in an associated cryostatic container. The direction of this static field is usually, in most magnets, in a direction indicated at 10 which is directed along the longitudinal body axis of the patient 1. The gradient coil 5 is composed of a GX coil that provides a field gradient in a x-axial direction, a GY coil that provides a field gradient in a Y-axial direction, and a GZ coil that provides a field gradient in a Z-axial direction. The respective coils are connected to excitation power sources 11, 12 and 13. The excitation power sources 11, 12 and 13 are connected to a central control unit 14. The RF coil 3 is composed of a transmission coil and a reception coil which are respectively connected to a RF oscillation apparatus 15 and a RF reception apparatus 16, both of which are connected to the central control unit 14. The central control unit 14 is connected to a display/operation console 17, whereby necessary information can be imaged and while at the same time, the operations of the system can be controlled.
Next, a description will be set forth as to the operations of the conventional magnetic resonance system with the above-described configuration.
To obtain whole-body cross-sectional images of the patient 1, a field-homogeneous space 18 which is created by the whole body magnet 6 is provided in the form of a large 40-50 cm sphere having a high homogeneity of less than or equal to 50 ppm. For this reason, the magnet 6 is required to be quite large, and, for example, in the case of a superconducting magnet, the magnet is 2.4 m in length, 2 m in width, 2.4 m in height and 5 to 6 tons by weight.
Even when such a large magnet is utilized the homogeneity within the 40-50 cm field-homogeneous sphere reaches a value of several hundred ppm at most. Thus, in order to reduce this value (and thus increase the homogeneity) to a value less than 50 ppm, the shim coil 4 is used for field corrections. When magnetic resonance imaging is performed, a portion of the patient to be diagnosed is brought into the field-homogeneous space 18. Then a radio frequency magnetic field generated by the RF oscillation apparatus 15 is applied through the RF coil 3 in a direction perpendicular to the static field direction 10 so as to stimulate resonance of desired nuclei such as hydrogen, for example, within a body cell of the patient. At the same time, field gradients are respectively generated by means of the excitation power sources 11, 12 and 13 and the gradient coils GX, GY and GZ coils.
The pulse sequences of the RF coil 3 and the gradient coil 5 are determined depending upon the nature of the illness effecting the patient and the image processing methods, so as to select the optimum method for analysis.
These pulse sequence operations are controlled by the central control unit 14. When the gradient field and the RF field are applied, in response thereto, magnetic resonance signals are generated from inside the patient 1. These signals are received and amplified by the RF coil 3 and the RF reception apparatus 16, and fed to the central control unit 14, wherein image processing of the data is performed so as to display the desired cross-sectional images on a CRT (cathode-ray tube) of the display/operation console 17.
However, a number of disadvantages exist in the conventional magnetic resonance system with the above-described configuration.
First, in order to realize a 40-50 cm field-homogeneous sphere, a huge whole body magnet is necessary. As a consequence, large manufacturing costs are required, and the sales price of the whole system including diagnosis apparatus is too large for most hospital and research center users
Further, due to the large-size and heavy weight of the magnet, the system cannot generally be installed within the existing diagnosis space but rather requires room enlargement and floor reinforcements or the building of wholly new physical facilities all of which further add to the already large capital expense.
The abovementioned disadvantages have prevented magnetic resonance system from gaining widespread use.
A second major disadvantage arises in the case of performing spectroscopy In spectroscopy systems it is necessary to create a very high field homogeneity of 0.1 ppm within a strong and uniform field of 1.5 to 2.0 Tesla. These parameters necessitate the use of a superconducting magnet. However, in general, it is difficult to change field strengths with a superconducting magnet because, as described above, superconducting magnets are operated in a perpetual current mode such that the current lead conductor is removed. For this reason, there are provided plural devices for use with the superconducting magnets such as to establish field strengths in the range of 0.2 to 0.6 Tesla when imaging hydrogen nuclei, and in the range of 1.5 to 2.0 Tesla when performing spectroscopy of phosphorus nuclei.
Therefore, it is difficult to utilize a single system to selectively perform imaging of hydrogen and phosphorus nuclei.
Yet a third major disadvantage of conventional systems is that the patient is completely enveloped within a magnet bore which is an extremely limited space, and thus the patient often suffers from claustrophobia.